1. Field of the Invention
The present invention relates to a semiconductor wafer for forming a semiconductor device and, more particularly, a solid state imaging device and a method of manufacturing the semiconductor wafer.
2. Description of the Prior Art
As a semiconductor wafer for forming a semiconductor device, a CZ substrate grown by a CZ method, an MCZ substrate grown by an MCZ method, an epitaxial wafer obtained by forming an epitaxial layer on the surface of the CZ or MCZ substrate, or the like has been conventionally used.
On the other hand, although the step of forming a semiconductor device is performed in an ultra clean room of class 100 or less, the semiconductor substrate cannot be completely prevented from being contaminated by impurities from gases, water, and apparatus for manufacturing a semiconductor device and the like. In addition, the dose of impurity implanted into the semiconductor wafer in the step of forming an epitaxial layer on the surface of the semiconductor substrate is larger than the dose of impurity implanted into the semiconductor wafer in the step of forming a semiconductor device.
When an impurity or crystal defects are present in the active region of the semiconductor wafer, the quality and characteristics of the semiconductor device are considerably degraded. In addition, when the impurity and crystal defects are present in the semiconductor wafer, the semiconductor wafer is easily damaged by radiation such as .alpha. rays, and the quality and characteristics of the semiconductor device are further degraded by this damage.
In order to remove the impurity and crystal defects from the active region, intrinsic gettering (IG) or extrinsic gettering (EG) has been conventionally performed. FIGS. 1 and 2 show the characteristics of a semiconductor device formed on an epitaxial wafer subjected to these processes described above.
In order to obtain the results shown in FIGS. 1 and 2, epitaxial layers were simultaneously formed on a CZ substrate which was not subjected to gettering, a CZ substrate which was subjected to EG and a CZ substrate which was subjected to IG, respectively. In this case, the EG was performed such that a polysilicon film having a thickness of 1.5 .mu.m was formed on the lower surface of the CZ substrate by a CVD method at a temperature of 620.degree. C. The IG was performed such that oxygen was precipitated by sequentially performing annealing at 1,100.degree. C. for 1.5 hours, annealing at 650.degree. C. for 10 hours and annealing at 1,050.degree. C. for 2 hours for the CZ substrate to form crystal defects inside the CZ substrate.
A MOS capacitor having a gate electrode consisting of an Al film and a gate insulating film consisting of an SiO.sub.2 film having a thickness of 20 nm and a CCD imaging device were formed on each of these epitaxial wafers. FIG. 1 shows a generation life time which is obtained by a C-t method using the MOS capacitor and represented as a value normalized such that a measurement value of a CZ substrate is set to be 1. FIG. 2 shows the number of white defects of each CCD imaging device represented as a value normalized such that a measurement value of an MCZ substrate is set to be 1. Note that these white defects are equivalent to dark currents caused by impurities or the like.
However, as is apparent from FIGS. 1 and 2, even when the EG or IG is performed for an epitaxial wafer, the generation life time of the epitaxial wafer is almost equal to that of a CZ substrate. The number of white defects of the epitaxial wafer cannot be reduced to that of an MCZ substrate. On the other hand, in the CZ or MCZ substrate, defects are present in not only the substrate but also the gate insulating film formed on the surface of the substrate, and current leakage caused by a decrease in breakdown voltage of the gate insulating film or an increase in interface state causes a transfer failure or the like in the CCD imaging device.